Also, just wondering, how much power can an OPA549 deliver? The datasheet didn't say

Well, since this isn't specifically an audio amplifier, I am not surprised that power output ratings for speaker loads (4 ohm, 8 ohm) are not given in the datasheet. However, if you look at the Total Harmonic Distortion graph, you will see several sets of lines, for different output power levels, and a note that the load is 4 ohms. One of the lines is for 75 W output. Note how the distortion rises sharply with frequency, even at low power output levels.

Actually, it is quite normal to see and increase in THD with frequency.

But check out the 3886. It's THD+N vs. Frequency curve is plotted with power out = 60 W into 4 ohms. At 20 kHz, the number is about 0.025%. For the 3875, with 40 W into 8 ohms, it is about 0.07%. And with the OPA549, at 75 W (or at 10 W) into 4 ohms it is about 0.2%. So what concerns me isn't that the THD increases with frequency, but how much it increases. The distortion in the 3886 increases by a factor of 10 from low frequencies to high frequencies. The distortion in the OPA549 increases by a factor of 100 from low frequencies to high frequencies.